Synthesis gas (“syngas”) typically contains trace nitrogen-containing compounds, principally ammonia and hydrogen cyanide. Other reactive nitrogen compound species, such as cyanogen and nitrogen oxides, may also be present in very small amounts. Collectively, these nitrogen-containing compounds are referred to herein as N-contaminants.
N-contaminants arise from the presence of one or more nitrogen-containing species in the feed to the synthesis gas generator. For example, N2 may be present in: (1) the feed natural gas; (2) the O2 feed after air separation for an oxygen blown syngas generation process; and/or (3) the air or oxygen-enriched air feed for an air blown process. In addition to or alternatively to these sources of N2, nitrogen-containing hydrocarbon species (especially for liquid and/or solid syngas generation feedstocks, such as residual oil or coal) may also be present in the syngas generator. The concentration of N-contaminants produced in the syngas generator may also be increased substantially through the recycle of Fischer-Tropsch tail gas into the syngas generation process. Similarly, the concentration of N-contaminants produced in the syngas generator may also be increased by recycling of tail gases from other processes into the syngas generator.
Virtually all commercially practiced and proposed syngas generation processes operate at extremely high temperatures, generally in the range of 1500°–2500° F., where the majority of the chemical reactions occur near or at chemical thermodynamic equilibrium. Under these conditions, small quantities of hydrogen cyanide (HCN) and ammonia (NH3) are typically produced. Yet smaller amounts of other reactive nitrogen-containing compounds, such as cyanogen, may also be produced. The amounts of HCN and NH3 in a syngas depends strongly on both the nitrogen concentration in the syngas generator feed and the process conditions, particularly pressure and temperature. Typical concentrations of these nitrogen-containing compounds in the syngas generator outlet stream which has not been further processed (referred to herein as a “raw synthesis gas”) are in the range from about 1 to about 50 vppm HCN and from about 5 to about 1000 vppm NH3. Generally, the raw syngas contains between about 10 and about 30 times more NH3 than HCN.
Ammonia, which is basic, is very soluble in water. Raw syngases contain both carbon dioxide and water vapor and at least about 90 wt % of the ammonia present in the raw syngas can be removed by cooling the raw synthesis gas to less than about 200° F. and condensing the produced water. CO2 dissolved in the condensed water will facilitate dissolution of the ammonia from the synthesis gas. The amount of ammonia in the syngas may be further decreased by use of a water scrubber.
HCN, on the other hand, is much less water soluble than NH3, and is somewhat acidic in solution. Therefore, HCN is much more difficult to remove by means of raw synthesis gas water knockouts and/or subsequent scrubbing. Removal by water scrubbing requires relatively large quantities of water, typically greater than 1:1 water:syngas mass ratios. Incremental HCN removal can be realized by recirculating the ammonia-containing wash water, produced by scrubbing the ammonia from the raw syngas which contributes to HCN disassociation and removal by water scrubbing. However, HCN removal with water scrubbing is inefficient, requiring excessive amounts of water in relation to the HCN quantity removed. A large number of known processes for HCN removal from synthesis gases, including HCN adsorption, catalytic conversion of HCN (hydrogenation and/or hydrolysis), and chemically treated water scrubbing of HCN are known. Other processes attempt to prevent the formation of HCN by upstream removal of N2 from natural gas. Such known processes, however, result in or require increased plant capital and/or operating costs, supply and disposal of treatment chemicals, and/or potential contamination of the treated synthesis gas. Moreover, may of these processes are hampered by the presence of other acidic materials, e.g. CO2.
Removal of HCN and NH3 from syngas is considered important because these nitrogen-containing compounds are poisons of Fischer-Tropsch catalysts, particularly non-shifting catalysts, and more particularly, those Fischer-Tropsch catalysts containing cobalt.